Bloodwynd was trapped, somehow, and MM took his place until he could be freed.
That finally happened shortly after the Death Of Superman, in the same issue Blue Beetle came out of his coma. In the next issue of JLA, the whole story was revealed, and the real Bloodwynd joined the team for a bit, I think.
Bloodwynd's membership in the League is questionable. He hung out with the team for a while, but he actually covered up Dreamslayer's alliance with the Overmaster that would lead to the end of the world in exchange for Dreamslayer not targeting him, and then sort of just faded out of the League as it became clear that they were going to conflict with Dreamslayer. The man's pretty amoral for a mystic, which makes him interesting, but he's certainly not League material.